1. Field of the Invention
This invention relates to card frames, and is more particularly directed to side plates thereof.
2. Description of the Prior Art
A `card frame` (also known as a `sub-rack`) is a kind of frame structure used to house electrical circuitry mounted on `cards` which, for example, comprise circuit boards, for instance printed circuit boards, mounting circuit components. A typical card frame comprises a pair of side plates spaced apart by generally horizontally extending cross rails. The cross rails typically mount card guides for receiving the cards together with connectors for making connections to the cards. The side plates and cross rails are of an electrically conductive material, in practice metal, and at least in some applications the conductive continuity between the cross rails and side plates should be of high quality to enhance screening and/or earth continuity. The cross rails and side plates are therefore generally secured together by screws or the like.
It is important that the cross rails be secured to the side plates in such a manner as to prevent the cross rails twisting, that is to say rotating about their longitudinal axes. (If, for example, this were not the case, the card guides or connectors could become dislodged or broken.) Some protection against twisting can be provided by securing each end of each cross rail to the associated side plate with two spaced screws instead of one. (See, for example, U.S. Pat. Nos. 3,184,069 and 4,002,381). Such a solution makes construction and assembly more complicated. In any event, it does not ensure positive location of the cross rails and side plates against twisting whereby the possibility of twisting cannot entirely be ruled out. For instance, some twisting could take place if one of the two screws were not properly tightened.
It is known from published UK Patent Application No. GB 2 035 703 A to fit a plastics moulding between each end of a cross rail and the associated side plate. The moulding has projections that, on one side, fit into a row of holes in the side plate and, on the other side, into recesses in the end of the cross rail, whereby an anti-twist mounting can be obtained with a single screw. However, the use of an extra component, namely a fairly complex plastics moulding, at each end of each cross rail increases cost and complexity. Moreover, it may in some cases detract from the quality of the conductive continuity between the cross rails and the side plates.
It is further known from published German Patent Application No. DE 2 653 468 A to provide groups of projections extending from the inside of each side plate to cooperate with recesses in the ends of the cross rails to prevent twisting of the cross rails. Assembly may be difficult since the recesses and projections must be accurately aligned. Moreover, alternative mounting positions for the rail must be spaced to such an extent that the rail will not, in one position, interfere with the projections of another group.
The side plates are often formed from steel plate, the plates having front mounting flanges which are integral with the plates or are in the form of angle members secured thereto. It is also known for the side plates to comprise aluminium extrusions. The direction of extrusion is vertically of the side plate in order that the mounting flange can be formed integrally therewith. Consequently, with such a side plate, no surface configuration extending in a generally front to rear direction can be formed since such a direction is of course transverse to the direction of extrusion.